Re: [PATCH v1 0/9] do not use s_dirt in ext4

On Thu, 2012-03-22 at 11:33 +0100, Jan Kara wrote:

  Then we have ext4_mark_super_dirty() call from 4 places - I forgot about
these originally... I kind of miss their purpose. Originally they were used
so that we write total number of free blocks and inodes in the superblock
but when we do not maintain them in the journal mode I don't see a reason
to periodically sync them in no-journal mode. Ted, what is the purpose of
these calls?

I do not understand what's the fundamental difference between journal
and non-journal mode. Why when we do have the journal we do not mark the
super-block as dirty in many places (e.g., in 'ext4_file_open()' - if we
do have the journal, when do we make sure we save the mount point path
change?).

May be it has something to do with behaving like the ext2 driver when
mounting ext2-formatted media with the the ext4 driver?

Jan, since Ted did not answer, may be you can figure out the reasons
from this commit message, which actually introduced the
'ext4_mark_super_dirty()' function?


commit a0375156ca1041574b5d47cc7e32f10b891151b0
Author: Theodore Ts'o [off-list ref]
Date:   Fri Jun 11 23:14:04 2010 -0400

    ext4: Clean up s_dirt handling
    
    We don't need to set s_dirt in most of the ext4 code when journaling
    is enabled.  In ext3/4 some of the summary statistics for # of free
    inodes, blocks, and directories are calculated from the per-block
    group statistics when the file system is mounted or unmounted.  As a
    result the superblock doesn't have to be updated, either via the
    journal or by setting s_dirt.  There are a few exceptions, most
    notably when resizing the file system, where the superblock needs to
    be modified --- and in that case it should be done as a journalled
    operation if possible, and s_dirt set only in no-journal mode.
    
    This patch will optimize out some unneeded disk writes when using ext4
    with a journal.
    
    Signed-off-by: "Theodore Ts'o" [off-list ref]

-- 
Best Regards,
Artem Bityutskiy